The present invention relates generally to interference mitigation in wireless communication systems and in particular to interference mitigation by training of interfering signals in wireless communication systems employing multiple input receive units.
Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity. Numerous system layouts and communications protocols have been developed to provide coverage in such wireless communication systems.
Currently, most wireless systems are broken up into separate coverage areas or cells. Typically, each cell has a base station equipped with an antenna for communicating with mobile or stationary wireless devices located in that cell. A cellular network consists of a number of such cells spanning the entire coverage area. The network has an assigned frequency spectrum for supporting communications between the wireless devices of subscribers and base stations in its cells. One of the constraints on a wireless communication system is the availability of frequency spectrum. Hence, any wireless system has to be efficient in using its available frequency spectrum.
It is well-known that attenuation suffered by electromagnetic wave propagation allows wireless systems to re-use the same frequency channel in different cells. The allowable interference level between signals transmitted in the same frequency channel determines the minimum separation between cells which can be assigned the same frequency channel. In other words, frequency channel re-use patterns are dictated by the amount of Co-Channel Interference (CCI) seen by the receiving unit (either the base station or the wireless subscriber device).
Clearly, high spectral efficiency is a desirable system characteristic. By reducing CCI the C/I ratio can be improved and the spectral efficiency increased. Specifically, improved C/I ratio yields higher per link bit rates, enables more aggressive frequency re-use structures (closer spacing between cells re-using the same frequency channels) and increases the coverage of the system.
It is known in the communication art that receiving stations equipped with antenna arrays, rather than single antennas, can improve receiver performance. Antenna arrays can both reduce the effects of multipath fading of the desired signal and suppress interfering signals or CCI. Such arrays can consequently increase both the range and capacity of wireless systems. This is true for instance of wireless cellular telephone and other mobile systems.
In mobile systems, a variety of factors cause signal corruption. These include interference from other cellular users within or near a given cell. Another source of signal degradation is multipath fading, in which the received amplitude and phase of a source varies over time. The fading rate can reach as much as 200 Hz for a mobile user traveling at 60 mph at PCS frequencies of about 1.9 GHz. In such environments, the problem is to cleanly extract the signal of the user being tracked from the collection of received noise, CCI, and desired signal portions summed at the antennas of the array.
In Fixed Wireless Access (FWA) systems, e.g., where the receiver remains stationary, signal fading rate is less than in mobile systems. In this case, the channel coherence time or the time during which the channel estimate remains stable is longer since the receiver does not move. Still, over time, channel coherence will be lost in FWA systems as well.
Antenna arrays enable the system designer to increase the total received signal power, which makes the extraction of the desired signal easier. Signal recovery techniques using adaptive antenna arrays are described in detail, e.g., in the handbook of Theodore S. Rappaport, Smart Antennas, Adaptive Arrays, Algorithms, and Wireless Position Location; and Paulraj, A. J. et al., xe2x80x9cSpace-Time Processing for Wireless Communicationsxe2x80x9d, IEEE Signal Processing Magazine, November 1997, pp. 49-83.
Some of the techniques for increasing total received signal power use weighting factors (for broadband cases these are filters such as space-time or space-frequency combiners) to multiply the signal recovered at each antenna of the array prior to summing the weighted signals. Given that antenna arrays offer recognized advantages including greater total received signal power, a key issue is the optimal calculation of the weighting factors used in the array. Different approaches to weight generation have been presented in the art.
If the channels of the desired and interfering signals are known, the weight generation technique that maximizes the signal-to-interference-plus-noise ratio (SINR), as well as minimizes the mean squared error (MMSE) between the output signal and the desired output signal, is the well-known Weiner-Hopf equation:
w=[Rxx]xe2x88x921rxd,
where rxd denotes the crosscorrelation of the received signal vector x with the desired signal, given by:
rxd=E[x*d],
where d is the desired signal, and Rxx is the received signal correlation matrix, which in turn is defined as:
Rxx=E[x*xT],
where the superscript * denotes complex conjugate and T denotes transpose.
Of course, this technique is only one of many. Other prior art techniques known in the art include joint detection of signal and interferers, successive interference canceling as well as space-time or space-frequency filtering and other techniques. More information about these techniques can be found in the above-cited references by Theodore Rapapport and Paulraj, A. J., as well as other publications.
Interference mitigation including CCI reduction for the purpose of increasing spectral efficiency of cellular wireless systems particularly adapted to a system using adaptive antenna arrays has been addressed in the prior art. For example, U.S. Pat. No. 5,819,168 to Golden et al. examines the problem of insufficient estimation of CCI and noise in communication channels which leads to an inability to suppress interference. In particular, Golden teaches to solve the problems associated with correct estimation of the Rxx correlation matrix by an improved strategy for determining the weighting coefficients to modify Rxx based on the ratio of interference to noise.
U.S. Pat. No. 5,933,768 to Skxc3x6ld et al. addresses the problem of interference suppression with little knowledge of the interfering signal. This is done by detecting a training sequence or other portion of the interfering signal whose structure is unknown to the receiver, estimating the interferer channel and using this information in a joint demodulation receiver. The training sequences come from a finite set of known training sequences. Typically, interferer channel is calculated for all training sequences from the finite set and the best value selected based on residual interference. Matters are further complicated because the training sequences of the interferers arrive at the receiver at undetermined times. The channel estimation is thus performed user by user and results in poor channel estimates of the interferers since their training sequences can overlap the higher powered random data sequence of the desired user signal.
In yet another communication system as taught in U.S. Pat. No. 5,448,753 to Ahl et al. interference is avoided. This is done by coordinating the direction and transmission times of the beams such that they do not cross. In this manner interference between switched beams in a network and especially between beams from adjacent base stations can be avoided. A significant effort has to be devoted to coordination between the users and the base stations in this scheme. Additionally, this technique only works in areas where there is a significant proportion of line-of-sight (LOS) paths between transmitters and receivers.
Unfortunately, the above-discussed and other methods to improve spectral efficiency by CCI suppression in wireless systems including adaptive antenna array systems do not exhibit sufficiently high performance. Thus, it would be desirable to improve interference suppression in wireless systems using antenna arrays and specifically using multiple input receive units. In particular, it would be desirable to improve CCI suppression such that a higher rate of frequency re-use could be employed in wireless systems.
The present invention provides a method for identifying one or more undesired signals at a multiple input receive unit in a wireless communication system. In accordance with the method, the multiple input receive unit receives a desired signal or several desired signals and an undesired signal or several undesired signals. The desired signals are intended for the multiple input receive unit and the undesired signals are usually intended for a different receive unit. A separability condition between the desired signal and undesired signal is provided to ensure that the undesired signal can be identified at the multiple input receive unit.
The separability condition according to the invention can be provided for in several ways. In one embodiment the desired signal has a tone at which no information is transmitted or a zero tone for separability. It is advantageous that in this case the undesired signal have a first training pattern substantially at the zero tone, i.e., at the zero tone frequency of the desired signal. Further separability is ensured when the desired signal is provided with an initial training pattern and the undesired signal is provided with a zero tone substantially at the initial training pattern, i.e., at the frequency used by the initial training pattern.
In another embodiment of the invention, the initial training pattern of the desired signal and the first training pattern of the undesired signal are separable. This can be achieved when the initial and first training patterns are linearly independent at the receive unit. For example, the initial and first training patterns can be orthogonal to ensure separability. Additionally, the initial and first training patterns can be transmitted at a boosted power level to ensure good reception at the multiple input receive unit. The first training pattern is conveniently used to train the undesired signal to determine a channel matrix for the undesired signal. Likewise, the initial training pattern is used to train the desired signal and determined the channel matrix for the desired signal. Once the channel matrices are known the effects of the undesired signal can be mitigated, e.g., the undesired signal can be cancelled by the receive unit.
In the same or another embodiment the initial training pattern is transmitted at an initial carrier frequency fk. The first training pattern of the undesired signal is transmitted at an offset carrier frequency. In a particular example, when the communication system is an OFDM system the offset carrier frequency is fk+m, where m is an integer. The initial and offset carrier frequencies can be chosen in accordance with a scheme or randomly. It is convenient, however, when the offset carrier frequency is proximate the initial carrier frequency fk such that the channel does not experience considerable variation between these two carrier frequencies. Furthermore, in a particularly convenient embodiment linearly independent training patterns are used in conjunction with zero tones.
In yet another embodiment, the separability condition is provided by ensuring appropriate transmission of the desired and undesired signals. Specifically, these signals are transmitted such that they are received in substantial coherence at the multiple input receive unit. This can be achieved by coordinating signal transmission. Coherent reception is advantageously practiced in conjunction with the use of offset training patterns and zero tones.
In another embodiment, a second undesired signal received at the multiple input receive unit is provided with a second separability condition between the desired signal and the second undesired signal. The second separability condition ensures that the second and desired signals are separable at the multiple input receive unit and that the second undesired signal can be thus identified. In particular, separability can be ensured by providing the second undesired signal with a second training pattern separable from the initial training pattern of the desired signal at the receive unit. This second training pattern can be transmitted at a boosted power level. Further separability can be ensured by the use of zero tones and/or coordinated transmission of desired signal and second undesired signal to achieve substantially coherent reception.
The second training pattern is conveniently used for training the second undesired signal. In some cases the undesired signal is a strong interfering signal and has to be mitigated, e.g., canceled for proper reception of the desired signal at the receive unit. At the same time, the second undesired signal is typically a weak interfering signal and it may not require direct mitigation. However, in cases where the second undesired signal is not sufficiently weak or as desired by the system designer, the second undesired signal is trained using the second training pattern. The channel matrix is then derived and used for mitigation.
The method of invention further includes training of the undesired signals at the multiple input receive unit. For this purpose the training patterns used are separable from each other. It is also convenient that no data is transmitted at the carrier frequencies on which training patterns are transmitted. In fact, it is most convenient when zero tones are assigned such that any carrier frequency is used for transmitting a training pattern of only one signal while other signals have zero tones at this carrier frequency.
A wireless communication system of the invention identifies undesired signals by using the separability condition. The system has transmit units which transmit signals, including desired and undesired signals. A control and processing device (e.g., central system control) or a collection of devices provide the separability condition between the desired and undesired signals. A receive processing unit at the multiple input receive unit identifies the undesired signal. The transmit units can include local transmit units and remote transmit units.
For example, the local transmit unit is a base station transceiver of a local cell in which the multiple input receive unit is located. The local transmit unit transmits the desired signal to the receive unit. The remote transmit units are base station transceivers of remote cells and their transmitted signals are undesired signals, e.g., strong interfering signals to the multiple input receive unit located in the local cell. Some more remote transmit unit transmit second undesired signals, e.g., weak interfering signals. The transmit units can be multiple output transmit units, thus creating a MIMO (Multiple Input Multiple Output) communication system.